It is well established that the motor activity pattern regulates physiological and molecular processes in skeletal muscle. It is our objective in this study to perform the first comprehensive and integrated analysis of the adaptive responses that occur in all 3 mammalian skeletal muscle fiber types with muscle use and disuse. Wistar female rats will be divided into groups to characterize the effects of (1) hindlimb immobilization produced by casting, (2) chronic electrical stimulation of hindlimb muscles, and (3) exercise-training. A novel approach to be used in a separate series of experiments will assess the effects of chronic electrical stimulation (i.e. motor activity) on rat hindlimb muscles during and following immobilization. The effects of exercise-training (treadmill running) on muscle recovery from immobilization will also be studied. At selected time periods after the initiation of the above experiments, muscle function will be evaluated by studying the isometric and isotonic contractile properties of selected muscles representative of the 3 mammalian fiber types. Biochemical studies will also be performed on myosin and the sarcoplasmic reticulum (SR). Myosin and the SR will be extracted from Type I, IIA, and IIB muscle samples. The myosin light and heavy chains will be analyzed using polyacrylamide gel electrophoresis, and functional studies on myosin ATPase conducted. The SR will be evaluated to determine Ca ions uptake, binding kinetics and the ATPase activity of this membrane system. Fiber type differences in the myosin and SR structure and function will be evaluated, the effect of use and disuse determined, and the results correlated with the physiological findings.